In semiconductor devices, the principal way of reducing contact resistance between an interconnect line and a silicon surface such as a polysilicon gate or silicon source/drain regions is by forming a metal silicide atop the polysilicon or silicon surface to be contacted, prior to application of the conductive film used to form the various conductive interconnect lines. Low resistivity and ease of formation are important considerations in selecting the metal silicide material. Presently, common metal silicide materials include but are not limited to CoSi2, TaSi2, and TiSi2. In today's semiconductor fabrication industry, silicides are typically formed by the so called salicide (self-aligned silicide) process. In the salicide process, a thin layer of metal is blanket deposited over the semiconductor substrate, including over exposed silicon surfaces such as source/drain regions, gate electrode regions, and other contact regions. The wafer (i.e., semiconductor substrate) upon which the semiconductor device is being formed, is then subjected to one or more high temperature annealing steps. The annealing process causes the metal to selectively react with the exposed silicon thereby forming a metal silicide, XSi. The process is referred to as a self-aligned silicide process because the silicide layer is formed only where the metal material directly contacts the exposed silicon area. In other areas where the metal film is separated from silicon such as by a dielectric layer, no silicide reaction occurs.
A preferred metal silicide in today's ultra-large scale integration (ULSI) semiconductor fabrication processes, is nickel silicide. Nickel silicide contacts are advantageously used in contacts of reduced dimension in order to provide resistivity reductions and therefore increased device speed, and to take advantage of nickel silicide's low leakage or diffusion characteristics. One shortcoming associated with silicide formation processes in general and in nickel silicide processes in particular, is non-uniformity of the formed silicide. The non-uniformity includes non-uniform thickness of the NiSi and resistivity, Rs, deviation across the formed silicide film. A common cause for such non-uniformities is the premature silicidation that undesirably occurs during the nickel deposition process, and prior to the subsequent annealing process designed to form the silicide. This is true when a pure nickel silicide is formed and also true when a binary phase alloy Ni(X)Si is formed. When a binary phase alloy Ni(X)Si is formed, Ni spiking during deposition additionally results in a non-uniform distribution of the alloying additive X, throughout the film. The spiking is due to different diffusivity characteristics of the different atoms in the substrate. Spiking may occur to silicon in a silicon substrate or to substrates such as SiGe, Ge or Si on SOG. Moreover, premature silicidation may occur due to the suppression of the NiSi formation temperature as a result of the plasma-assist phenomena.
Moreover, chemical dry cleaning processes are increasingly being used to remove moisture from substrates. These dry cleaning processes require a subsequent heating operation to drive off the by-products. Such heating operations also cause reaction between Ni and Si to form NiSi, resulting in an uneven NiSi film.
It would therefore be desirable to provide a nickel silicide film or nickel alloy silicide film in which the silicide film is formed by a method in which the thermal budget of Ni deposition is carefully controlled and in which a silicide film with superior uniformity is formed during a subsequent controlled thermal operation designed to form the silicide.